Introduction to folds

Looking at different types of folds on seismic data

One limb is longer than the other.

These folds are asymmetrical.

This fold is symmetrical.

The limbs are the same length.

1.5 km N

Folds can be gentle, symmetrical and rounded.

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CMP

Fold axial plane

Or inclined, angular and asymmetric.

Fold axial plane

Look at this horizon

1.5 km N

There may be other deformation features associated with a fold.

Can you spot the faults?

What sort of faults are they?

⇃↾

⇃↾

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760.0780.0800.0820.0840.0860.0880.0900.0920.0940.0960.0980.01000.01020.01040.01060.01080.01100.01120.01140.01160.01180.01200.01220.01240.01260.01280.01300.01320.01340.01360.0- B92-40 -

1380.01400.01420.01440.01460.01480.01500.01520.01540.01560.01580.01600.01620.01640.01660.01680.01700.01720.01740.01760.01780.0SP:

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BP-84-195, 1330.85 B90-37, 399.04

80090010001100120013001400150016001700

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Reverse faults often occur with folds as both are caused by compressive forces.

⇃↾⇃⇂

But not all the faults are reverse faults. This normal fault was re-activated.

When is an antiform an anticline?

(…and when is a synform a syncline?)

antiform

antiform

antiformsynform

synform

Remember this?

In this series of folds the rocks have not been inverted, therefore the youngest overlie the oldest rocks.

anticline

anticline

anticlinesyncline

syncline

To be an anticline or a syncline the relative age of the rocks must be known.

anticline

syncline

Look at the anticline:

From the centre of the structure the age of the rocks increases towards the outside:

the youngest rocks are at the core.

From the centre of the structure the age of the rocks decreases towards the outside:

the oldest rocks are at the core.

oldest

youngest

Now look at the syncline:

oldest

youngest

⤚ ⤙

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• The individual properties of a rock and the physical setting (e.g. temperature and confining pressure) determine how it will react under stress.

• Some layers may be more competent than others and so deform in different ways, i.e. buckling, flexural slip or shearing (see links for more details).

• There are many causes of stress in rocks that can cause folds:

Plate movements e.g. orogenies

Igneous intrusions e.g. granite batholiths

Salt diapirs (where large quantities of low density salt rise towards the surface causing deformation).

• The time over which these stresses are applied can also affect the deformation e.g. low strain rates over extended time periods produces plastic deformation.

• Folds vary in size from microscopic to many km. Only the large scale folds can be seen on seismic data.

Overview